Add _line + _line_safe methods to MessageBuilder
Add _line + _line_safe methods to MessageBuilder

Add new methods to MessageBuilder to push content similar to the other
methods, except with the addition of appending a newline afterwards.

This should help prettify some MessageBuilder usage.
eLunate/serenity,acdenisSK/serenity,Roughsketch/serenity,zeyla/serenity,Lakelezz/serenity,Flat/serenity,zeyla/serenity.rs